1. Field of the Invention
The present invention relates generally to an apparatus and method for estimating a Carrier-to-Interference-and-Noise Ratio (CINR) in a wireless communication system. More particularly, the present invention relates to an apparatus and method for estimating a CINR in an Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) wireless communication system.
2. Description of the Related Art
Wireless communication systems have been developed to transmit radio signals to allow terminals to perform communication any place. The typical wireless communication system includes a Code Division Multiple Access (CDMA) cellular mobile communication system. The CDMA cellular mobile communication system fundamentally provides voice service and can additionally provide data service. However, with the rapid development of communication technology, the data service, compared with the voice service, is increasing in importance in the CDMA cellular mobile communication system. Due to the increase importance of the data service in the CDMA cellular mobile communication system, users and service providers desire to transmit a larger amount of data at a higher rate. However, the CDMA cellular mobile communication system is considered to have reached its limit in providing higher-speed data service due to limited resources.
To address the problems caused by the limited resources of the CDMA cellular mobile communication system, live discussions have occurred on standardization for the OFDM/OFDMA wireless communication systems, and commercialization thereof. The OFDM/OFDMA wireless communication system can transmit data at a high rate using a plurality of orthogonal sub-carriers. Herein, both the OFDM and OFDMA schemes, unless stated otherwise, will be referred to as an OFDMA scheme.
The OFDMA wireless communication system needs high-speed data transmission. For the high-speed data transmission, there is a need for high-order modulation schemes. Modulation schemes are divided into low-order modulation schemes such as Binary Phase Shift Keying (BPSK) and Quadrature Phase Shit Keying (QPSK), and high-order modulation schemes such as 16-ary Quadrature Amplitude Modulation (16 QAM) and 64 QAM. Performance of a transmission method based on the high-order modulation schemes greatly depends on channel conditions. That is, the transmission method can have a very high data rate in good channel conditions. However, in bad channel conditions, because many retransmissions are required, the use of the high-order modulation schemes, rather than the use of the low-order modulation schemes, may cause deterioration of the performance. Therefore, in the OFDMA wireless communication system, correctly detecting the channel conditions and using a modulation scheme appropriate for the detected channel conditions is important.
In a method for detecting the channel conditions by a transmitter of the wireless communication system, a receiver estimates a CINR for a particular signal transmitted from the transmitter and transmits the estimated CINR over a feedback channel, so the transmitter can detect the channel conditions. Here, the particular signal denotes a signal transmitted to a corresponding user. The transmitter may also determine a data rate using the information received over the feedback channel. The information received over the feedback channel has various usages. A description will now be made of a general method for estimating a CINR in the OFDMA wireless communication system.
FIG. 1 is a block diagram illustrating a structure of a terminal receiver with a CINR estimator in a general OFDMA system.
A signal received from an antenna ANT is applied to a radio frequency (RF) unit 110, and the RF unit 110 extracts a baseband analog signal from the received signal up-converted for transmission. The baseband analog signal output from the RF unit 110 is provided to an analog-to-digital converter (ADC) 120, and the ADC 120 converts the analog signal into a digital signal. The digital signal output from the ADC 120 is filtered by a filter 130 and then input to a Cyclic Prefix (CP) and serial-to-parallel (S/P) conversion unit 140. The CP and S/P conversion unit 140 removes a CP contaminated by multiple transmission paths, and converts the CP-removed serial digital signal into a parallel analog signal. The parallel signal undergoes Fast Fourier Transform (FFT) in an N-point (N-pt) FFT processor 150, so the time-domain input signal is converted into a frequency-domain signal. The frequency-domain signal is input to a signal synthesizer 170.
A pseudo-random noise (PN) code generator 160 for generating a unique PN code allocated to every user generates a unique PN code allocated to the receiver itself, and outputs the generated PN code to the signal synthesizer 170. The signal synthesizer 170 synthesizes the PN code uniquely allocated to the corresponding user with the frequency-domain signal, so the receiver can extract only the signal transmitted thereto. The signal extracted by the signal synthesizer 170 is divided into two signals, where one signal is input to a CINR estimator 180 and the other signal is input to a channel estimator 190. The CINR estimator 180 estimates a ratio of a desired signal in the received signal to an undesired noise component included in the received signal. The channel estimator 190 estimates a change in channel and channel conditions.
The CINR estimated in the receiver is transmitted to a transmitter over a feedback channel as described above. The transmitter determines a modulation order using the feedback information, modulates data in the determined modulation order, and transmits the modulated data to the receiver. Assuming that a terminal communicates with a base station #l, a signal obtained after CP removing in the receiver of FIG. 1 can be expressed asy[n]=hl[n]ΘNsl[n]+i[n]+w[n]  (1)
In Equation (1), ΘN denotes N circular convolution, hl[n] denotes a time-domain channel response from the base station #l to the terminal, sl[n] denotes a transmission signal from the base station #l, w[n] denotes an additive white Gaussian noise (AWGN), and i[n] denotes an interference signal from an adjacent cell. In addition, a signal obtained after N-pt FFT operation in the receiver of the terminal can be expressed asy(k)=Hl(k)sl(k)+i(k)+w(k)  (2)
In Equation (2), l denotes an index of a base station, k denotes an index of a sub-carrier, Hl(k) denotes an N-pint Discrete Fourier Transform (DFT) value of hl[n] and is a frequency-domain channel response characteristic. In addition, w(k) and i(k) denote N-point DFT coefficients of time-domain AWGN noises w(n) and i(n), respectively. Herein, the sum w(k)+i(k) of interferences and noises is modeled with white noises having power
            I      l        N    ,where Il denotes power of interference signals from base stations, except for the base station #l in communication with the terminal, to the terminal. In the OFDMA wireless communication system, because signal transmission is performed through N sub-carriers, power of interference signals are also carried on the N sub-carriers, achieving 1/N scaling.
The notations used herein are defined as follows. An interference signal is expressed with a subscript l because it varies according to a reference base station, and an additive noise is expressed without any subscript because it is independent of the base station. Herein, [n] and (k) are used as factors for representing a pre-FFT signal, which is a time-domain signal, and a post-FFT signal, which is a frequency-domain signal, respectively. Assuming that |sl(k)|2=1, a CINR between the base station #l and the terminal is defined as
                              CINR          l                =                                            ∑                              k                =                0                                            N                -                1                                      ⁢                                                  ⁢                          E              ⁢                                                                                                            H                      l                                        ⁡                                          (                      k                      )                                                                                        2                                                          I            l                                              (        3        )            
Because |sl(k)|2=1, by multiplying y(k) by s*l(k) in Equation (2), it is possible to obtain a signal zl(k) by removing the original signal sl(k) from Equation (2), as given below.zl(k)=Hl(k)+il(k)+wl(k)  (4)
In Equation (4), il(k) and wl(k) denote interference signals and additive noises, respectively, and are values given by multiplying a received signal y(k) by s*l(k). In addition, because |sl(k)|2=1, power of il(k)+wl(k) is
            I      l        N    .
Generally, CINR estimation is achieved by the CINR estimator 180 of FIG. 1 in cooperation with the channel estimator 190. In brief, the CINR estimator 180 obtains an estimated channel value Ĥl(k) from the channel estimator 190, estimates carrier power (or signal power) using the estimated channel value Ĥl(k) in accordance with Equation (5) below, and estimates power of the interferences and noises using the estimated carrier power in accordance with Equation (6) below.
                                          C            ^                    l                =                              ∑                          k              =              o                                      N              -              1                                ⁢                                          ⁢                                                                                                        H                    ^                                    l                                ⁡                                  (                  k                  )                                                                    2                                              (        5        )                                                      I            ^                    l                =                                            ∑                              k                =                o                                            N                -                1                                      ⁢                                                  ⁢                                                                                                z                    l                                    ⁡                                      (                    k                    )                                                                              2                                -                                    C              ^                        l                                              (        6        )            
Using Equation (5) and Equation (6), the final estimated CINR can be given as
                                          C            ^                    ⁢                      INR            l                          =                                                            C                ^                            l                                                      I                l                            ^                                =                                                    ∑                                  k                  =                  o                                                  N                  -                  1                                            ⁢                                                          ⁢                                                                                                                                    H                        ^                                            l                                        ⁡                                          (                      k                      )                                                                                        2                                                                                      ∑                                      k                    =                    o                                                        N                    -                    1                                                  ⁢                                                                  ⁢                                                                                                                        z                        l                                            ⁡                                              (                        k                        )                                                                                                  2                                            -                                                ∑                                      k                    =                    o                                                        N                    -                    1                                                  ⁢                                                                  ⁢                                                                                                                                                  H                          ^                                                l                                            ⁡                                              (                        k                        )                                                                                                  2                                                                                        (        7        )            
The method for estimating a CINR using an estimated channel value in accordance with Equation (7) greatly differs in CINR performance according to channel estimation performance. That is, accurate channel estimation increases the CINR performance, but inaccurate channel estimation decreases the CINR performance. Because the transmitter determines a modulation order depending on an estimated CINR fed back from the receiver, the inaccurate CINR estimation causes deterioration in the total system performance and unnecessary repetition of retransmission. In addition, because interference and noise power is involved in the process of calculating signal power (or carrier power) in accordance with Equation (4), a bias caused by the interference and noise power may occur in the calculated signal power. That is, the interference and noise power may be included in the signal power in the calculation process, making it difficult to calculate an accurate CINR.
Accordingly, there is a need for an improved apparatus and method for calculating an accurate CINR that increases transmission without deteriorating a system performance.